Study On Microstructures And Wear Properties Of The Coatings Fabricated On Titanium Alloy By Laser Alloying With Nitrogen Gas, Boron And Graphite Powders

The excellent properties such as higher specific tensile strength and corrosion resistance of pure titanium and its alloys cannot be exploited fully due to their intrinsic weakness such as lower hardness and poor wear resistance. So, in the present research, laser-alloying technique is employed to surface modify pure titanium and its alloys for improving their wear resistance.Laser-alloying parameters such as output power, scanning speed, beam diameter, etc. have a great effect on the alloyed layers. Under the present experimental conditions (the category of precoated powers and their thickness, the shield gases and their pressure, etc.), the desirable alloyed layers can be obtained with the output power of 1200W, the beam size of 2 mm and the scanning speed of 3.0~4.0mm/s.The results of laser-carburizing pure titanium and Ti-6A1-4V show that the morphology of in situ formed dendritic TiC crystals is composed of granules in a linear arrangement. According to the law of natural selection and the fittest survive, the direction normal to the {111} close-packed planes should be the preferential growth direction because the atoms are easier attached to them. So, during the solidification, the liquid atoms preferentially attach to the {111} close-packed planes, which results in the growth of primary truck along 〈111〉 crystallographic direction. With the primary truck growing, large amounts of latent heat are released which reduces the degree of supercooling in the vicinity of the primary truck and thus induces the second branches to grow laterally. Since the direction normal to the 〈111〉 crystallographic direction is affected slightly by the latent heat, the secondary branches grow laterally along the fittest direction parallel to the {111} crystal plane. During the solidification, the temperature gradient in the vicinity of the crystal nucleus is not equal to each other in different direction and the solidification always conducts along the direction in which the temperature gradient decrease sharper. Thus the final growth result of the TiC crystal is in the form of dendrite.According to the equations of continuous growth and lateral spiral growth, crystal growth model can be described by the degree of critical supercooling △T_C which separates the growth models into two regimes:△T_k<△T_c, lateral growth and a faceted crystal structure; T_k>△T_c, continuous growth resulting in non-faceted dendritic crystals. On the other hand, the previous experiment showed that with the decrease of solute concentration the △T_C needed for the continuous growth of crystal embryos increases. So, the growth model of TiC is continuous growth at the beginning of solidification because of high degree of supercooling. With the...